This invention relates to a siphon coffee maker for extracting coffee by making use of the siphon phenomenon.
Siphon coffee makers provided with a control unit incorporating a microcomputer for automatic extracting and warmth retaining operations are well known. Such a siphon coffee maker comprises a lower container employed for containing water and heated by an electric heater such as a sheathed heater, an upper container employed for containing coffee powder and communicated to the interior of the lower container through a water feed tube, and a control unit for executing a series of operations by energizing and deenergizing the sheathed heater. More specifically, when a start switch is manually operated with the upper and lower containers filled with a desirable amount of coffee powder and the corresponding amount of water respectively, an extracting operation command is supplied to the control unit, which operates to energize the heater. Heat is applied to the lower container and the water contained therein is heated and boiled. The hot water is fed from the lower container to the upper container through the water feed tube owing to well known siphon phenomenon, thereby extracting coffee. When the heater is deenergized, the extracted coffee in the upper container is fed to the lower container through the water feed tube and reserved therein. Subsequently, the heater is re-energized so that a warmth retaining operation is performed for extracted coffee reserved in the lower container.
In performing the above-mentioned warmth retaining operation, the heater is generally needed to be controlled so as to be energized and deenergized in accordance with degree of temperature increase of the lower container. However, the heater is disposed in the vicinity of the lower container in the siphon coffee maker and the heater of a relatively large output type is employed for the reason that the water is needed to be boiled in a short period. Accordingly, it is difficult to accurately detect the temperature of the lower container during the warmth retaining operation. In view of the problem, the prior art siphon coffee maker has not been provided with a function of detecting the temperature of the lower container and instead, it is contemplated that the heater is controlled so as to be intermittently energized at predetermined intervals during the warmth retaining operation.
However, since a constant electrical output is supplied to the heater during the warmth retaining operation in the conventional construction of the siphon coffee maker, coffee in the lower container is likely to be excessively heated in the case that a small amount of coffee is reserved in the lower container, whereby the coffee is caused to be reboiled or the flavor of coffee is degraded. While, when a large amount of coffee is reserved in the lower container, heat is likely to be absorbed into the circumference, which results in a disadvantage that a sufficient warmth retaining does not take place even though heat is applied to the lower container by the heater.
Further, since the lower container is subjected to the ambient heat as well as heat generated by the heater when the room temperature is high, coffee in the lower container is likely to be overheated or the flavor thereof is degraded. Additionally, when the room temperature is relatively low, heat of coffee in the lower container is likely be absorbed into the circumference, also resulting in a disadvantage that a sufficient warmth retaining does not take place even though heat is applied to the lower container by the heater.